Composites are heterogeneous structures comprising two or more components, the combination taking advantage of the individual properties of each component as well as synergistic effects if relevant. Advanced composites refer to a class of materials in which engineered (e.g., man-made) fibers are embedded in a matrix, typically with the fibers being aligned or even woven such that a material with directional (anisotropic) properties is formed. A common example of an advanced composite is graphite-epoxy (Gr/Ep) wherein continuous aligned carbon fibers (stiff/strong/light) are embedded in a polymer (epoxy) matrix. Materials such as these have been used in the Stealth Bomber and Fighter and in sporting equipment, among other applications. Advanced composite systems comprising multiple materials can also be useful in applications where performance benefits from weight savings.
Combining carbon nanotubes (CNTs) with other materials, including macro-advanced composites, can create yet new materials with enhanced physical properties, particularly enhanced engineering properties. Specifically, CNTs have been studied and applied widely as reinforcements for polymers. CNTs have been shown to exhibit strong adhesion to several polymers, for example, where individual CNTs are embedded in and then pulled out of a thermoplastic. While the use of CNTs in composite materials has been studied, existing CNT processing techniques often display several drawbacks. For example, the syntheses of CNTs often result in structures having large diameter and insufficient length, which often results in poor alignment of the CNT axes. Also, dispersion of the CNTs in secondary materials, which typically requires uniform wetting of the CNTs by the secondary materials, is often hindered by CNT agglomeration. Last, alignment of CNTs in the secondary materials may be difficult to achieve in general, particularly when alignment of nanotubes is desired in a system comprising large (e.g., orders of micron diameter) advanced fibers, a secondary material (matrix), and well-aligned CNTs in the secondary material. There are numerous examples of composites comprised of disordered arrangements and/or low volume fractions of CNTs, which exhibit one or more of these drawbacks.
Accordingly, improved materials and methods are needed.